Conventionally, there has been known an electric power steering apparatus which produces a steering assist torque so as to assist a driver's steering operation. Such an electric power steering apparatus detects a steering torque acting on a steering shaft by use of a torque detection apparatus, computes a target assist torque which increases as the steering torque increases, and feedback-controls the amount of electricity supplied to an electric motor so as to obtain the calculated target assist torque. Accordingly, in such an electric power steering apparatus, in particular, the torque detection apparatus must be reliable.
A steering torque detection apparatus detects the torsion angle of a torsion bar provided in a steering shaft, and calculates a steering torque which is in proportion to the torsion angle. For example, a torque detection apparatus disclosed in Japanese Patent Application Laid-Open (kokai) No. 2003-315182 employs a configuration which detects the torsion angle of a torsion bar by making use of two resolvers. In this torque detection apparatus, a first resolver is provided at one end of the torsion bar, and a second resolver is provided at the other end thereof. Steering torque is detected from the difference between the rotational angle (θ1) detected by the first resolver and the rotational angle (θ2) detected by the second resolver.
Each resolver includes an excitation coil to which an excitation AC signal is supplied and which energizes a rotor coil, and a pair of detection coils fixedly disposed around the torsion bar. The pair of detection coils are assembled with a shift of 90 degrees (π/2) (electrical angle) provided therebetween. One detection coil outputs an AC signal having an amplitude corresponding to the sin value of the rotational angle of the rotor, and the other detection coil outputs an AC signal having an amplitude corresponding to the cos value of the rotational angle of the rotor.
The two resolvers are connected to an ECU, which constitutes a torque computation section. The ECU supplies an excitation AC signal to the excitation coil of the first resolver and the excitation coil of the second resolver via a common excitation line connected to one end of each excitation coil. Also, the ECU receives a sin value detection signal and a cos value detection signal from the respective detection coils of the first resolver via individual detection lines connected to one ends of the detection coils. Similarly, the ECU receives a sin value detection signal and a cos value detection signal from the respective detection coils of the second resolver via individual detection lines connected to one ends of the detection coils. Furthermore, the other ends of the excitation coil and the detection coils are connected to the ECU via a common ground line.
The ECU computes the rotational angles of the torsion bar at positions where the respective resolvers are provided from the outputs signals from the pair of detection coils of each resolver. The ECU then detects a steering torque acting on the torsion bar from the difference between the two rotational angles.